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anext() 函数

anext() 函数从异步迭代器中返回下一个元素。

复杂度参考

操作 时间 空间 备注
anext() 调用 O(1) O(1) 外加一次 __anext__() 调用本身；对于 async def 或异步生成器，它只会构建可等待对象
等待结果 O(k) O(1) k = 一次 __anext__() 步骤的工作量，包括其中的 await；空间不含该步骤自身的分配
带默认值 O(k) O(1) 迭代耗尽时返回默认值；每次挂起额外 O(1)

手写的可等待对象与默认值

传入默认值时，anext() 会包装 __anext__() 返回的可等待对象，并在这次 await 的每一步都调用它的 __await__()，而不是只调用一次。async def __anext__() 或异步生成器会从上次停下的地方继续并正常完成。而 __await__() 是生成器函数的类每次调用都会从头开始；如果每次调用都挂起，这个 await 就永远不会完成。对于这样的迭代器，请不要传入默认值，而是自行捕获 StopAsyncIteration。

基本用法

获取下一个异步元素

import asyncio

async def async_generator():
    for i in range(3):
        yield i

async def main():
    async_iter = aiter(async_generator())

    # Get next item - O(k) where k = async iterator work for this step
    value = await anext(async_iter)
    print(value)  # 0

    value = await anext(async_iter)
    print(value)  # 1

asyncio.run(main())

使用默认值

import asyncio

async def async_generator():
    yield 1
    yield 2
    # Iterator exhausted after

async def main():
    async_iter = aiter(async_generator())

    print(await anext(async_iter))  # 1
    print(await anext(async_iter))  # 2

    # Default when exhausted - O(1) once iterator is exhausted
    value = await anext(async_iter, "END")
    print(value)  # "END"

asyncio.run(main())

不带默认值（抛出异常）

import asyncio

async def async_generator():
    yield 1

async def main():
    async_iter = aiter(async_generator())

    print(await anext(async_iter))  # 1

    try:
        # No default - raises StopAsyncIteration when exhausted (O(1) at exhaustion)
        print(await anext(async_iter))
    except StopAsyncIteration:
        print("Iterator exhausted")

asyncio.run(main())

实际示例

import asyncio

async def fetch_items(items):
    for item in items:
        await asyncio.sleep(0.1)  # Simulate async work
        yield item

async def main():
    # Create async iterator - O(1)
    iterator = aiter(fetch_items(["a", "b", "c"]))

    # Get items manually - O(k) each where k depends on iterator body
    first = await anext(iterator)
    print(f"First: {first}")  # First: a

    second = await anext(iterator, None)
    print(f"Second: {second}")  # Second: b

    # Get remaining with loop
    async for item in iterator:
        print(f"Item: {item}")  # Item: c

asyncio.run(main())

与 next() 的比较

import asyncio

# next() - synchronous - O(1) call, the generator's step runs inside it
def sync_gen():
    yield 1
    yield 2

sync_iter = iter(sync_gen())
print(next(sync_iter))  # Synchronous

# anext() - asynchronous - O(1) call, awaiting depends on generator
async def async_gen():
    yield 1
    yield 2

async def main():
    async_iter = aiter(async_gen())
    print(await anext(async_iter))  # Must await

asyncio.run(main())

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